1. Field of the Invention
The present invention relates to a power conversion circuit for providing a specific voltage waveform to an inductive load such as a squirrel cage induction motor.
2. Background Art
Examples of power conversion circuits for providing a specific voltage waveform to an inductive load such as a motor include inverters. Japanese Laid-Open Patent Publication No. H06-165510 discloses an inverter circuit having two half-bridge circuits. The half-bridge circuits include a high side switching device and a low side switching device connected in series across a power supply, a first diode connected in antiparallel with the high side switching device, and a second diode connected in antiparallel with the low side switching device. The first diode provides a freewheeling path for the stored energy discharged from the load (inductive load) when the low side switching device is turned off. The second diode, on the other hand, provides a freewheeling path for the stored energy discharged from the load when the high side switching device is turned off.
In such power conversion circuits, the two potentials of the DC power supply are alternately supplied to the external load with a suitable timing, thereby controlling the average load voltage and the average load current.
When the low side switching device is on, a current flows through the series circuit including the power supply, the load (inductive load), and the low side switching device. Then when the low side switching device is turned off from the on state, the current flowing through the inductive load does not immediately fall off, but tends to continue flowing. The result is that a current flows through the load and the first diode in a continuous loop. This current is referred to as a “freewheeling current.”
When the freewheeling current is flowing in the first diode, if an on signal is applied to the gate of the low side switching device, the low side switching device gradually turns on. However, the first diode (through which the freewheeling current is flowing) does not immediately turn off and remains on until all the stored carriers in the diode are removed; that is, the first diode conducts in both the forward and reverse directions during that period. Thus, if the low side switching device is turned on when the first diode is in the conducting state, the power supply will be short circuited by the first diode and the low side switching device. This phenomenon is referred to as the “recovery” of the diode. The instantaneous short circuit current is referred to as “recovery current.”
Since the recovery current is large and flows through the low side switching device, the current increases the switching loss of the switching device. This increase in the switching loss due to the recovery of the diode occurs regardless of the number of power supply voltages and the number of half-bridge circuits in the power conversion circuit.